Analysis of CD or DVD Discs and Stampers using Scanning Probe Microscopy (SPM) from NT-MDT

Nowadays, Digital Versatile Discs (DVD) and Compact Discs (CD) are popular data storage. The so called pit is the information unit of CD/DVD. CD and DVD are manufactured by stamping a polycarbonate. Generally, nickel stampers are used as a stamp that contains bumps, which, in turn, form pits. The quality of a one stamper decides the quality of the group of discs; in other words, preliminary control of stamper is essential.

SPM for Pit Analysis of CDs and DVDs

Magnetic properties of nickel stamper and its large size (140 mm) make it challenging for electron microscopy control. In addition, devices are available for surface analysis based on electric principles, but such measurements cannot visualize pit geometry. SPM is a perfect tool for pit geometry analysis, thanks to its high resolution, high measurement rapidity and possibility of non-destructive measurements of large samples (Figure 1). The use of the SPM for manufacturing the stamper for CD/DVD is a perspective method of quality monitoring. SPMs assist in checking the stamper quality and in mitigating the risk of defect appearances. The topography changes of the stamper can be investigated under various external influences, such as observation of the deformation due to blow or polishing. Moreover, it is possible to observe topography changes caused by heating during the pressing of a polycarbonate. Several parts of stamper surface are controlled during manufacturing, and quality of the stamper is appreciated. Additionally, selective control of discs is executed.

Figure 1. Single pit of CD measured by SPM.

Observing the Magnetization Reversal Process

The experiments in an external magnetic field allow one to observe the magnetization reversal processes. The design of the NT-MDT SPMs makes it possible to apply the various external magnetic fields during magnetic force microscopy (MFM) measurements. The SOLVER P47 equipped with electromagnet was used to obtain the present results. It is capable of producing magnetic fields up to 500Oe. The studied sample was cobalt polycrystalline patterned film (40 nm thick), which was deposited on high-oriented pyrolytic graphite in the form of the micron size rectangles (Figure1).

Equipment and Methods

As observed above, the possibility of the measurements of the large sample is the main demand of the CD/DVD industry to SPM. NT-MDT devices have three basic configurations that meet this demand and provide effective analyzing of CD/DVD:

SPM Solver P7LS (Figure 2). The optical viewing system, the motorized positioning stage, the vacuum holder for samples with the size up to 300 mm in diameter, and automatic approach make this device most suitable for CD/DVD industry.

SPM Stand Alone SMENA in tandem with special designed SMENA base for large sample (Figure 2). This device can provide the same noise level as Solver P7LS. Such design doesn't include motorized positioning stage and vacuum holder. Automatic approach is available, if required. Sample positioning is performed manually.

SPM Stand Alone SMENA with special long leg enables placing the CD/DVD between them.

In this case, the user has to provide hard fastening of the sample under tip.

Figure 2. The NT-MDT SPM Solver P7LS.

Modes of Operation for Topography Measurements

There are two main modes for topography measurements: semi-contact and contact. Semi-contact mode uses cantilever oscillating at its resonance frequency. Consequently, the sample and tip are in contact only for a small part of the oscillation period. This results in a substantial reduction in the destructive action of the friction and capillary forces. Thus, semi-contact mode is more appropriate for soft materials, for example, polycarbonate.

Whisker Type Cantilevers

NT-MDT manufactures "Whisker type" cantilevers that offer more accurate measurements when compared to standard silicon cantilevers. This type of cantilever has a standard silicon cantilever augmented by carbon needle with high aspect ratio. The carbon needle is grown on the tip end by Focused Ion Beam (FIB). "Whisker type" cantilever are appropriate for more precision measurements of abrupt steps. Further, as carbon is a hydrophobic material, the adsorption layer of water is absent on the carbon tip. As a result, the accuracy of the measurements is increased.

Statistical Treatment of Data

The obtained results require statistical treatment. NT-MDT software contains a menu of tools called "Grain analysis" (Figure 3), which was developed for the statistical treatment of the particles lying on flat surface (also suitable for pits). By using these tools, one can determine the geometrical size of pits, approximate pits by different figures, such as ellipse, rectangular, and rectangular with rounded sides (imitation of CD/DVD pits). Based on such approximation, the direction of axes, the angles between them, and other parameters are determined.

Figure 3. Main view of the menu "Grain analysis".

Parameters Determining CD/DVD Stamper Quality

CD/DVD parameters for checking the key test characteristics, which establish the quality of either CD/DVD or stamper, are:

Pit (bump) size and pit (bump) form. For instance, pits (bumps) must have a flat area (Figure 4a). If such a flat surface is not observed (Figure 4b), then an error can occur during reading. The depth of the pits is a vital parameter due to its effect on the amplitude of signal during reading. The SPM Solver P7LS can also measure the pit height with resolution of a fraction of nanometer

Ratio of pit volume to volume of single pit. This is a significant technology characteristic of CD/DVD.

Number of pits per area unit, that is, data density

All these parameters can be computed by the NT-MDT software for the investigated area.

Figure 4. AFM images of the surface of the nickel stampers made by different technologies. Images obtained by semi-contact mode, SOLVER P7LS.

Surface Quality and Surface Defects

In addition, SPM checks the quality of the CD/DVD surface by exposing the surface defects. Figure 5 shows two scratch and knoll.

Figure 5. SPM image of defective surface of the CD disc.

Stamper Topography

The knowledge of the either bump or pit parameters quantified at different places of the sample enables effective controlling of the CD/DVD manufacture. The topography of a stamper is shown in Figure 6. After measuring the topography, the menu "grain analysis" was applied to acquire topography data. Black lines shown in Figure 7 mark contour of bumps at preset levels; red lines are approximation of bumps by ellipse. The different parameters were computed for the bump array. Some of them are shown in Table 1. The quality of data reading is influenced by these defects.

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